Poisons do not stack and their effect is calculated 1 round after. Poisons run over each other. You can use own backlash poison as "antidote" against worser poisons, or if opponent happens to have 2 different poisons you can intentionally lose to pick up the weaker.

VERSION 2
Round 1 : Opponent goes in with Karrion, poison and you at 7
Round 2 : Opponent hits you for 3 damage, Karrions poison eats away 2 more - you are at 2 HP.
Round 3 : You win with Gork. Karrions poison hits in for this round, you lose. Technically Gorks poison replaces Karrions poison, but since its effect would be counted next round it does not save you.